Irritable male syndrome

Irritable male syndrome (IMS) is an annual behavior pattern that has been described in Soay sheep and other mammals with a strictly seasonal breeding pattern and described in a 2002 literature review of animal behavior by Lincoln A. Gerald. IMS is a striking feature in mammals with seasonal breeding patterns; it manifests at the end of the mating season.[1]

Soay sheep mate for five weeks during November and December each year, and give birth five months later in the spring.[2] The rams' testosterone levels soar during the late autumn mating season. In the winter, testosterone levels fall and they stop mating. As their testosterone levels were falling, the rams became more nervous and withdrawn, striking out irrationally.[1]

Gerald concluded his 2002 review, writing: "This brief review challenges the dogma that male mammals are constantly reproductively active following sexual maturity at puberty. Males of many, and perhaps the majority of, long-lived species, express periodic changes in testicular activity and behaviour during their normal life cycle. In the most extreme examples, as illustrated by the Soay ram, males continually cycle between the sexually active/fertile state and the sexually inactive/infertile state, often on an annual or long-term basis. In individuals, testicular activity may vary with changes in social status, nutrition, health, age and other factors. This also applies to man."[1]